Recently, demand on stereochemically pure compounds is increasing rapidly. One important use of these pure stereoisomers is as synthetic intermediates in the pharmaceutical industry. For example, it is becoming gradually evident that enantiopure drugs possess many advantages over racemic drug mixtures. The advantages often include less side effects and better efficacies of the enantiopure compounds [see, e.g., Stinson, S.C., Chem Eng News, Sep. 28, 1992, pp. 46-79].
For example, triadimenol can exist as four isomers. The (−)-(1S,2R)-isomer has a stronger activity than the (+)-(1R,2R)-isomer and the (−)-(1S,2S)-isomer has a stronger activity than the (+)-(1R,2S)-isomer. Among the four isomers of dichlorobutrazol, the (1R,2R)-isomer is known to have stronger activity. Also, for etaconazole, the (+)-(2S,4S)- and (−)-(2S,4R)-isomers are known to have better fungicidal effect than others.
Therefore, if only one isomer having higher activity can be prepared selectively, better effect can be achieved with less amount and, accordingly, environmental pollution resulting from the use of the chemicals can be decreased. Especially for drugs, if one of the isomers exhibits toxicity in human, it is very important to selectively prepare one isomer only.
Accordingly, in medicine-, pharmacy- and biochemistry-related fields, preparation of optically pure compounds for improving medicinal effect or preventing side effects is a very important task.
However, still many drugs are used as racemic compounds with unavoidable side effects owing to the existence of undesirable enantiomers (see, e.g., Nguyen, et al., Chiral Drugs: An Overview, Int. J. Biomed. Sci., 2 (2006) 85-100). A few techniques are available for the preparative or analytical scale chiral separations. However, it costs immense time and efforts to find out a separation technique suited for the racemates of interest. Even if one should succeed in resolving an enantiomer, he will then face the next difficulty, i.e., to enable the chiral resolution on an industrial scale.
For example, the efficacies of pure stereoisomers of vanilloid antagonists including N-[4-(1-aminoethyl)-phenyl]-sulfonamide derivatives have been elucidated [e.g., WO 2008-013414 A1, WO 2007-133637 A2, WO 2007-129188 A1, WO 2010-010934 A1].
As a method for synthesizing single isomers of the N-[4-(1-aminoethyl)-phenyl]-sulfonamide derivatives, asymmetric synthesis using the Ellman's reagent is known. For example, WO 2008-013414 A1, WO 2007-133637 A2, WO 2007-129188 A1 and WO 2010-010934 A1 present a method of obtaining desired stereoisomers by introducing the Ellman's reagent and inducing asymmetric reduction using the same. However, this method is disadvantageous in that a low-temperature reaction condition should be maintained to achieve high optical purity (enantiomer excess, % ee). Also, the process is dangerous because excessive generation of hydrogen and heat occurs when the reaction is terminated. In addition, the disposal cost of the excessively produced organic and inorganic wastes is also disadvantageous in terms of economy.